The removal of matter suspended in wastewater is an important aspect of industrial wastewater treatment. The final step of removal is usually filtration. To be successful, the solid matter to be filtered must exhibit good "filterability"; among other characteristics, filterability requires that the material not be sticky or tacky, i.e., that the solid particles not adhere to one another. Many methods have been proposed and used for "detackification" of materials that are otherwise hard to filter.
For example, in the printed circuit board industry, a dry film polymer is commonly used as a "photoresist" mask in the printed circuit board patterning and etching process. When rinsed, this photoresist material eventually becomes a constituent of an aqueous wastewater stream that usually also contains amounts of dissolved copper, tin, and sometimes lead. Traditional treatment of this wastewater first entails the addition of a detackifier that is usually an aluminum-based product, followed by acidification to below 5.0, in order to precipitate the aqueous polymer. The resulting precipitate is tacky and would be very difficult to filter if not for the aluminum detackifier, which forms an aluminum hydroxide precipitate that coats the tacky polymer.
If the filtrate contains unacceptable levels of copper and lead, a further metal precipitant that is functional at a pH of less than 5.0, normally a sulfide, is usually added. For example, U.S. Pat. No. 3,740,331 to Anderson discloses using sulfides in combination with iron to remove heavy metals from wastewater. However, Anderson does not suggest that such treatments could be used at high concentrations in order to detackify sludges and thereby improve their filterability. Moreover, most inorganic sulfides will generate hydrogen sulfide in this use, while some organic sulfides will generate carbon disulfide; both hydrogen sulfide and carbon disulfide are very toxic gases, and hazardous to workers. There are few precipitants suitable for this application. One product sold for this purpose is a Degussa product known as "TMT-15"; this is an organic sulfide, trimercaptotriacene, that is added at dosages on the order of 5000 ppm.
Commonly assigned U.S. Pat. Nos. 5,451,327 and 5,372,726, incorporated herein by this reference, disclose sulfide compositions that are extremely well suited for this application, because use of these compositions in this process provides the desired precipitation without emission of hydrogen sulfide.
However, even if the sulfide compositions disclosed by commonly assigned U.S. Pat. Nos. 5,451,327 and 5,372,726 are used to precipitate the metals in the photoresist-containing wastewater stream, the use of an aluminum salt as a detackifying agent has several further and inherent disadvantages. At a pH of 5.0, a significant amount of the aluminum re-dissolves and is discharged with the filtrate, possibly violating discharge limits. Further, the aluminum produces an aluminum hydroxide floc that is hydroscopic and difficult to dewater, adding significantly to the amount of sludge for disposal. The encapsulated water also slows the filtration time, increasing labor and capital equipment costs.
Proprietary powdered materials, such as those sold by the Morton Chemical Company under the trade name Dynachem, are also used to detackify photoresist wastewater. These materials, which are believed by the present inventor to contain aluminum, but are not otherwise defined, are added at an alkaline pH and then acidified to flocculate the aqueous photoresist polymer. However, these powdered materials do not remove metals to desired levels, are dusty to use, and produce large volumes of sludge, which are slow to filter.
A treatment process for photoresist wastewater which not only detackified the suspended solids but also removed the dissolved metals would provide a major benefit, particularly if the photoresist wastewater treatment process resulted in reduced sludge quantities and filtration times.
Under some circumstances it is not necessary to remove the photoresist polymer from the wastewater stream. More particularly, certain circuit board manufacturing facilities would be permitted by local regulation to discharge wastewater comprising photoresist polymer directly to the local Publicly Owned Treatment Works ("POTW") along with other water streams including biodegradable organic substances ("BOD" wastes), if the levels of copper and lead in their photoresist wastewater were sufficiently low. However, at present there is available no suitable method to precipitate and flocculate the copper and lead so that they can be removed by industrial filtration equipment. More particularly, the photoresist polymer prevents flocculation and "blinds" (i.e., clogs) any filter cloths fine enough to remove the copper and lead precipitates. It would be a major benefit to the printed circuit board industry if a photoresist wastewater treatment were provided capable of precipitating heavy metals, such that they could be removed by conventional filter "press" equipment without blinding the filter cloth.
A similar problem arises in connection with processing of wastewater streams resulting from commercial paint spraying operations as performed in spray booths. Regulations require the emissions of paint spray to be controlled, which is traditionally accomplished by spraying water into the paint "overspray" emissions. However, unless treated with detackifiers, particles of the usual solvent-based paints remain sticky and adhere to all surfaces in the treatment system. The sticky particles build up and become entrained in the spray booth water, which, if untreated, may cause plugging of piping and spray nozzles, thus reducing the efficiency of the spray booth operation and increasing its emission of volatile organic compounds ("VOCs"). Accordingly, a detackifying agent is normally used in the spray application of solvent-based paints such as polyurethanes, epoxy resins, lacquers, and enamels. The detackifying agents used in this service are typically based upon clays, amphoteric metals, polymers, or combinations of these.
By comparison, water-based paints are typically resin suspensions stabilized by a surfactant matrix; when mixed with the spray booth water, these materials disperse and/or emulsify, again resulting in an increase in both suspended and dissolved solids. While these solids must be coagulated and removed from the spray booth water in order to maintain an effective painting operation, these solids do not require detackification as required for organic solvent based paints. However, the solids in water-based paints nonetheless require agglomeration for effective removal from the system.
In both solvent-based and water-based paint systems, heavy metals (e.g., lead, copper, zinc, nickel, or chromium) may be provided for corrosion resistance, or may be contained in the paint pigments. Therefore, before discharge, these paint wastes may have to be treated with a metal precipitating agent to meet regulatory discharge limits. None of the present detackifying methods is effective in removing metals from the wastewater. For example, U.S. Pat. No. 5,843,337 to Mitchell discloses treatment of paint wastes using a chemically modified tannin. While possibly effective in coagulating and detackifying solid paint suspended in an aqueous system, this process does not remove heavy metals from the wastewater. Consequently, it would be a major benefit if a single water-treating process could both detackify and remove dissolved metals from the wastewater.
In some cases, both organic solvent-based paints and water-based paints are sprayed in the same paint spray booth. Since solvent based paints need detackification (a surface adsorption process), and waterborne paints need coagulation (a particle charge process), it would be a major benefit if a single water treating agent could both detackify and coagulate paint wastes of either type.
Water-based paints sometimes include oils that form further emulsions in the wastewater. These emulsions are sometimes difficult to break and therefore require an acidic treatment ("acid break") to separate the water from the oil. Availability of a water-treating agent capable of operation in an acid condition and having a high surface area to adsorb the separated oil would be a major benefit for processes operating under these conditions.